<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <div id="app">
    firstName: <input type="text" v-model="firstName">
    lastName: <input type="text" v-model="lastName">
    =>
    {{fullName}}
  </div>
  <script src="./vue.global.js"></script>
  <script>
    const { createApp } = Vue;
    const app = createApp({
      data(){
        return {
          firstName: '',
          lastName: ''
        }
      },
      computed: {
        fullName(){
          return this.firstName + '.' + this.lastName;
        }
      }
    })
    app.mount('#app')
  </script>
</body>
</html>